The present invention relates to the development of planographic printing plates, and especially lithographic printing plates having an imaged photosensitive coating with regions that are soluble in an alkaline developer solution.
Historically, the processing of positive plates involves immersion of the plate in a sump of developer as it passes through a plate processor. The plate then exits the sump, and is typically subjected to a rotating brush or molleton and a nip roll set prior to being rinsed. This method of development relies entirely on the chemical dissolution of the solubilized coating to produce the plate. The developer is an alkaline solution typically containing an alkali metal silicate and/or alkali metal hydroxide along with other wetting agent/surfactants. Positive plate development generally has only small latitude in the development variables. For example, the temperature of the developer is critical, and must be controlled over a small range. The development is influenced by any flow dynamics (e.g. even small eddy currents) in the sump. This presents difficulties to get uniform development or consistent development from plate to plate. It is also difficult to assure that shadows and background areas clean out without attacking highlight dots in halftones.
Negative working plates generally have greater latitude in the development variables. Latitude is in large part determined by the relative solubility between the imaged and nonimaged regions of the coating. In an ideal imaged plate, the relatively insoluble regions experience no dissolution over a wide range of immersion dwell time in the developer solution, whereas over the same wide range of dwell time the relatively soluble regions quickly and completely dissolve. The resulting plate can produce accurate shadows in regions where the relatively insoluble, oleophilic coating has not been dissolved, clean background in regions where the relatively soluble coating has been completely removed, revealing the underlying hydrophilic substrate, and highlights in regions where dots of coating material are closely interspersed with adjacent regions of completely dissolve coating.
In practice, this ideal is not achievable, because the developer solution chemistry, temperature, and dwell time must be traded off to optimize cleanout of background while retaining the small dots of coating that provide good highlights in the printed product. Such optimization typically requires that the developer solution be strongly reactive and the dwell time be of long enough duration, to dissolve all the background, while conditioning agents in the developer solution, such as surfactants, help protect the relatively insoluble regions of the coating from reacting with the reactive ingredient of the developer solution.